Christian Gaillard

Role of microbial mats in the fossilization of soft tissues

It has been speculated that microbial mats are an important agent in the fossilization of soft tissues, particularly when apatite (Ca 5 [CO 3 , PO 4 ] 3 [OH, F]) is involved. This has been tested by chemical analyses of the Jurassic limestones of Cerin, France, where phosphatized soft tissues are abundant and are associated with unequivocal microbial mats. The sedimentary distribution of P, K, and Fe following deposition was controlled by the presence of the mats. P concentrations in the mats may approach 2.5 times those elsewhere in the sediment. The highest P concentrations correlate with the occurrence of phosphatized soft tissues. This is the first analytical evidence to demonstrate a fundamental role for microbial mats in the preservation of soft-bodied fossils.

Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary

The major evolutionary events that characterize the Precambrian–Cambrian transition are accompanied by profound ecological changes in the composition of benthic communities, the nature of the substrate, and the occupation of marine ecospace. The increased animal activity on and within the substrate is attested to by numerous trace fossils, such as the cosmopolitan Treptichnus pedum whose fi rst appearance is used as the global stratotype section and point (GSSP) to mark the base of the Cambrian. In spite of its major importance in biostratigraphy, the maker of Treptichnus trace fossils, and more generally of treptichnids, has long remained an enigma. Treptichnids were subhorizontal burrow systems produced in the subsurface and had a worldwide distribution throughout the Cambrian. Here we show, by using experimental ichnology, that the treptichnid burrow systems were most probably produced by priapulid worms or by worms that used the same locomotory mechanisms as the Recent priapulids (e.g., Priapulus). Their typical three-dimensional morphology with repeated arcuate probing branches suggests that their function was related to the feeding strategy of the worm such as predation or scavenging upon small epibenthic or endobenthic invertebrates. This interpretation is strongly supported by the preserved gut contents of Cambrian priapulids from the Burgess Shale Lagerstatte that contain effectively a variety of small epibenthic prey. The antiquity of treptichnids would designate priapulids as one of the earliest infaunal colonizers of the substrate that possibly interacted with epibenthic communities, thus playing a leading role (1) in the construction of the early marine food chain, and (2) as important subhorizontal bioturbators in the early stages of the “Cambrian Substrate Revolution.”

Microbialite morphology, structure and growth: a model of the Upper Jurassic reefs of the Chay Peninsula (Western France)

Abstract During the Early Kimmeridgian, the northern margin of the Aquitaine Basin (Western France) is characterised by a significant development of coral reefs. The reef formation of the Chay Peninsula comprises two main reefal units, in which the microbial structures can contribute up to 70% of framework. The microbial crusts, which played an important role in the stabilisation and growth of the reef body, show the characteristic clotted aspect of thrombolitic microbialites. Corals are the main skeletal components of the build-ups. The bioconstructions of the Chay area are thus classified as coral-thrombolite reefs. Four main morpho-structural types of microbial crusts are distinguished: (1) pseudostalactitic microbialites on the roof of intra-reef palaeocaves; (2) mamillated microbialites, found either on the undersides or on the flanks of the bioherms; (3) reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units; and (4) interstitial microbialites in voids of bioclastic deposits. Thrombolitic crusts developed on various substrates such as corals, bivalves, or bioclasts. The thrombolites formed a dense, clotted and/or micropeloidal microbial framework, in which macro- and micro-encrusters also occur. Variations in accumulation rate strongly influenced the reef morphology, in particular its relief above the sediment surface. The coalescence of the coral-microbialite patches created numerous intra-reef cavities of metre-scale dimensions. The direction of microbial growth, which defined the macroscopic microbialite forms, strongly depended on the position within the reef framework but was also controlled by water energy, accumulation rate and light availability.

Jelly-falls historic and recent observations: a review to drive future research directions

The biological pump describes the transport of particulate matter from the sea surface to the ocean’s interior including the seabed. The contribution by gelatinous zooplankton bodies as particulate organic matter (POM) vectors (“jelly-falls”) has been neglected owing to technical and spatiotemporal sampling limitations. Here, we assess the existing evidence on jelly-falls from early ocean observations to present times. The seasonality of jelly-falls indicates that they mostly occur after periods of strong upwelling and/or spring blooms in temperate/subpolar zones and during late spring/early summer. A conceptual model helps to define a jelly-fall based on empirical and field observations of biogeochemical and ecological processes. We then compile and discuss existing strategic and observational oceanographic techniques that could be implemented to further jelly-falls research. Seabed video- and photography-based studies deliver the best results, and the correct use of fishing techniques, such as trawling, could provide comprehensive regional datasets. We conclude by considering the possibility of increased gelatinous biomasses in the future ocean induced by upper ocean processes favouring their populations, thus increasing jelly-POM downward transport. We suggest that this could provide a “natural compensation” for predicted losses in pelagic POM with respect to fuelling benthic ecosystems.

A Constructional Model for Zoophycos

SUMMARY: Zoophycos is a complex trace fossil. Hundreds of these fossils, very similar to the type ichnospecies, have been observed in Devonian to Cretaceous deposits. The detailed analysis of the characteristics of the traces has resulted in a detailed constructional model. The specimens of Zoophycos that we have studied were constructed upwards into the sediment. They are the result of a very efficient mining programme, corresponding to the sediment feeding activity of a supposedly sipunculid worm. The formation of lobes may be linked to the evolution of the trace-maker or to its ethology.

Lower carboniferous Zoophycosfrom the Tournai area (Belgium): Environmental and ethologic significance

The aim of this work is to describe and interpret well preserved Lower Carboniferous Zoophycos from Belgium. They are compared to other similar Zoophycos studied by the same authors from Jurassic and Lower Cretaceous from France. Finally, some general conclusions are proposed which are also supported by data from the literature. Lower Carboniferous deposits in the Tournai area (Belgium) are characterized by the abundance of well preserved Zoophycos previously described as Spirophyton. The upper unit of the Tournai Limestones, the Antoing Formation (Ivorian), was studied here. It is usually made of fine-grained carbonate mudstones or wackestones, occasionally passing to packstones. The paleoenvironmental setting is typical of the deep part of a carbonate ramp, with occasional storm deposition. The general organization of Tournaisian Zoophycos is the same as that seen in Jurassic and Lower Cretaceous specimens of southeastern France: a single open tube and a lamina with an upward helicoidal growth. This similar organization suggests a similar ethology for the corresponding tracemaker considered as a deep sediment feeder with an efficient mining program. The more simple morphology of the Tournaisian burrow system suggests that this program is less complex than for the Mesozoic equivalents. Zoophycos is best developed in fine-grained sediments. In opposition to the Jurassic examples of southeastern France, this sediment must have still been soft, never slightly firm, when colonized. In addition, the abundance of Zoophycos at the top of locally frequent storm deposits could indicate a fairly opportunistic behavior. Considering the above mentioned assumption, the increasing complexity of the morphology and the modification of the related ethology, a possible evolution from an r-strategy to a K-strategy is suggested for the Zoophycos-creating organism during geologic time.

Ecological Significance of the Arthropod Fauna from the Jurassic (Callovian) La Voulte Lagerstätte

The La Voulte Lagerstätte is remarkable for its unique soft-bodied fauna (e.g., worms, coleoid squids) and its exceptionally preserved arthropods mainly found in small sideritic concretions. This arthropod fauna includes 30 different species assigned to the crustaceans, the thylacocephalans and the pycnogonids. Crustaceans are the most diversified group with 23 species distributed in a dozen families. Quantitative analyses based on 388 nodules reveals four dominant groups: (i) the enigmatic thylacocephalan arthropods (33%), (ii) the Solenoceridae shrimps (22%), (iii) the Coleiidae crustaceans (15%), and (iv) the Penaeidae shrimps (10%). Converging lines of evidence from depositional environment and modern analogues, indicate that this arthropod fauna probably inhabited a deep water setting most probably exceeding 200 m (= bathyal zone) under dysphotic or aphotic conditions. This new set of data sheds new light on the deep-sea colonisation by animal communities in the Mesozoic.

TRACE FOSSILS FROM NEARSHORE TO OFFSHORE ENVIRONMENTS: LOWER DEVONIAN OF BOLIVIA

Abstract Abundant and diverse trace fossils occur in deposits, recording an Early Devonian transgression in Bolivia. The very well-exposed Presto-El Peral section is located in the Interandean Belt 50 km northeast of Sucre. Trace fossils can be assigned to five associations from nearshore to offshore environments (namely Skolithos, Diplocraterion, Altichnus, Palaeophycus, and Zoophycos associations). They clearly illustrate the different colonization phases of the benthos with increasing marine influence. A deeper-water Helminthopsis association occurs in other complementary sections on the Eastern Cordillera. Variation of bioturbation intensity, ichnodiversity, and preservation potential is evaluated. The morphology of traces, mainly deep burrows, shows a significant gradient related to the behavior of tracemakers and the environmental changes. Main trends are consistent with the Seilacherian bathymetric model which is detailed here and/or emended with a well-exposed field example. A precise correlation is proposed with fossil benthic associations dominated by brachiopods. The whole gives an accurate model for benthos colonization and benthic marine zonation.

A giant Upper Jurassic turtle revealed by its trackways

Exceptional fossilization of large tetrapod swimming traces occurs in the Cerin Lagerstatte (Jura Mountains, France). These trackways are imprinted in Jurassic (Late Kimmeridgian) lagoonal fine-grained limestones and are attributed to giant turtles, which swam with a simultaneous movement of their forelimbs like the modern ones. These turtles swam in very shallow waters close to land, perhaps near a nesting area. As a major consequence, these new ichnologic data place the origin of true large marine turtles during the Jurassic period and not during the Cretaceous period as previously considered on the basis of skeletal remains.

The Paleoecology, Habitats, and Stratigraphic Range of the Enigmatic Cretaceous Brachiopod Peregrinella

Modern and Cenozoic deep-sea hydrothermal-vent and methane-seep communities are dominated by large tubeworms, bivalves and gastropods. In contrast, many Early Cretaceous seep communities were dominated by the largest Mesozoic rhynchonellid brachiopod, the dimerelloid Peregrinella, the paleoecologic and evolutionary traits of which are still poorly understood. We investigated the nature of Peregrinella based on 11 occurrences world wide and a literature survey. All in situ occurrences of Peregrinella were confirmed as methane-seep deposits, supporting the view that Peregrinella lived exclusively at methane seeps. Strontium isotope stratigraphy indicates that Peregrinella originated in the late Berriasian and disappeared after the early Hauterivian, giving it a geologic range of ca. 9.0 (+1.45/–0.85) million years. This range is similar to that of rhynchonellid brachiopod genera in general, and in this respect Peregrinella differs from seep-inhabiting mollusks, which have, on average, longer geologic ranges than marine mollusks in general. Furthermore, we found that (1) Peregrinella grew to larger sizes at passive continental margins than at active margins; (2) it grew to larger sizes at sites with diffusive seepage than at sites with advective fluid flow; (3) despite its commonly huge numerical abundance, its presence had no discernible impact on the diversity of other taxa at seep sites, including infaunal chemosymbiotic bivalves; and (4) neither its appearance nor its extinction coincides with those of other seep-restricted taxa or with global extinction events during the late Mesozoic. A preference of Peregrinella for diffusive seepage is inferred from the larger average sizes of Peregrinella at sites with more microcrystalline carbonate (micrite) and less seep cements. Because other seep-inhabiting brachiopods occur at sites where such cements are very abundant, we speculate that the various vent- and seep-inhabiting dimerelloid brachiopods since Devonian time may have adapted to these environments in more than one way.